.\"
.\" Copyright (c) 2008	     Los Alamos National Security, LLC  All rights reserved.
.\" Copyright (c) 2008-2009  Sun Microsystems, Inc.  All rights reserved.
.\"
.\" Man page for ORTE's Hostfile functionality
.\"
.\" .TH name     section center-footer   left-footer  center-header
.TH ORTE_HOSTS 7 "Jun 10, 2020" "4.0.4" "Open MPI"
.\" **************************
.\"    Name Section
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.SH NAME
.
ORTE_HOSTS \- OpenRTE Hostfile and HOST Behavior: Overview of OpenRTE's support for user-supplied
hostfiles and comma-delimited lists of hosts
.
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.\"    Description Section
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.SH DESCRIPTION
.
.PP
OpenRTE supports several levels of user-specified host lists based on an established
precedence order. Users can specify a \fIdefault hostfile\fP that contains a list of
nodes available to all app_contexts given on the command line. Only \fIone\fP default
hostfile can be provided for any job. In addition, users
can specify a \fIhostfile\fP that contains a list of nodes to be used for a specific
app_context, or can provide a comma-delimited list of nodes to be used for that
app_context via the \fI-host\fP command line option.
.sp
The precedence order applied to these various options depends to some extent on
the local environment. The following table illustrates how host and hostfile directives
work together to define the set of hosts upon which a job will execute
in the absence of a resource manager (RM):
.sp
.nf
 default
 hostfile      host        hostfile       Result
----------    ------      ----------      -----------------------------------------
 unset        unset          unset        Job is co-located with mpirun
 unset         set           unset        Host defines resource list for the job
 unset        unset           set         Hostfile defines resource list for the job
 unset         set            set         Hostfile defines resource list for the job,
                                          then host filters the list to define the final
                                          set of nodes available to each application
                                          within the job
  set         unset          unset        Default hostfile defines resource list for the job
  set          set           unset        Default hostfile defines resource list for the job,
                                          then host filters the list to define the final
                                          set of nodes available to each application
                                          within the job
  set          set            set         Default hostfile defines resource list for the job,
                                          then hostfile filters the list, and then host filters
                                          the list to define the final set of nodes available
                                          to each application within the job
.fi
.sp
This changes somewhat in the presence of a RM as that entity specifies the
initial allocation of nodes. In this case, the default hostfile, hostfile and host
directives are all used to filter the RM's specification so that a user can utilize different
portions of the allocation for different jobs. This is done according to the same precedence
order as in the prior table, with the RM providing the initial pool of nodes.
.sp
.
.\" **************************
.\"    Relative Indexing
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.SH RELATIVE INDEXING
.
.PP
Once an initial allocation has been specified (whether by an RM, default hostfile, or hostfile),
subsequent hostfile and -host specifications can be made using relative indexing. This allows a
user to stipulate which hosts are to be used for a given app_context without specifying the
particular host name, but rather its relative position in the allocation.
.sp
This can probably best be understood through consideration of a few examples. Consider the case
where an RM has allocated a set of nodes to the user named "foo1, foo2, foo3, foo4". The user
wants the first app_context to have exclusive use of the first two nodes, and a second app_context
to use the last two nodes. Of course, the user could printout the allocation to find the names
of the nodes allocated to them and then use -host to specify this layout, but this is cumbersome
and would require hand-manipulation for every invocation.
.sp
A simpler method is to utilize OpenRTE's relative indexing capability to specify the desired
layout. In this case, a command line of:
.sp
mpirun -pernode -host +n1,+n2 ./app1 : -host +n3,+n4 ./app2
.sp
.PP
would provide the desired pattern. The "+" syntax indicates that the information is being
provided as a relative index to the existing allocation. Two methods of relative indexing
are supported:
.sp
.TP
.B +n<#>
A relative index into the allocation referencing the <#> node. OpenRTE will substitute
the <#> node in the allocation
.
.
.TP
.B +e[:<#>]
A request for <#> empty nodes - i.e., OpenRTE is to substitute this reference with
<#> nodes that have not yet been used by any other app_context. If the ":<#>" is not
provided, OpenRTE will substitute the reference with all empty nodes. Note that OpenRTE
does track the empty nodes that have been assigned in this manner, so multiple
uses of this option will result in assignment of unique nodes up to the limit of the
available empty nodes. Requests for more empty nodes than are available will generate
an error.
.sp
.PP
Relative indexing can be combined with absolute naming of hosts in any arbitrary manner,
and can be used in hostfiles as well as with the -host command line option. In addition,
any slot specification provided in hostfiles will be respected - thus, a user can specify
that only a certain number of slots from a relative indexed host are to be used for a
given app_context.
.sp
Another example may help illustrate this point. Consider the case where a user has a default
hostfile containing:
.sp
.nf
dummy1 slots=4
dummy2 slots=4
dummy3 slots=4
dummy4 slots=4
dummy5 slots=4
.fi
.sp
.PP
This may, for example, be a hostfile that describes a set of commonly-used resources that
the user wishes to execute applications against. For this particular application, the user
plans to map byslot, and wants the first two ranks to be on the second node of any allocation,
the next ranks to land on an empty node, have one rank specifically on dummy4, the next rank
to be on the second node of the allocation again, and finally any remaining ranks to be on
whatever empty nodes are left. To accomplish this, the user provides a hostfile of:
.sp
.nf
+n2 slots=2
+e:1
dummy4 slots=1
+n2
+e
.fi
.sp
.PP
The user can now use this information in combination with OpenRTE's sequential mapper to
obtain their specific layout:
.sp
.nf
mpirun --default-hostfile dummyhosts -hostfile mylayout -mca rmaps seq ./my_app
.fi
.sp
.PP
which will result in:
.nf
.sp
rank0 being mapped to dummy3
.br
rank1 to dummy1 as the first empty node
.br
rank2 to dummy4
.br
rank3 to dummy3
.br
rank4 to dummy2 and rank5 to dummy5 as the last remaining unused nodes
.sp
.fi
Note that the sequential mapper ignores the number of slots arguments as it only
maps one rank at a time to each node in the list.
.sp
If the default round-robin mapper had been used, then the mapping would have resulted in:
.sp
.nf
ranks 0 and 1 being mapped to dummy3 since two slots were specified
.br
ranks 2-5 on dummy1 as the first empty node, which has four slots
.br
rank6 on dummy4 since the hostfile specifies only a single slot from that node is to be used
.br
ranks 7 and 8 on dummy3 since only two slots remain available
.br
ranks 9-12 on dummy2 since it is the next available empty node and has four slots
.br
ranks 13-16 on dummy5 since it is the last remaining unused node and has four slots
.fi
.sp
.PP
Thus, the use of relative indexing can allow for complex mappings to be ported across
allocations, including those obtained from automated resource managers, without the need
for manual manipulation of scripts and/or command lines.
.
.
.\" **************************
.\"    See Also Section
.\" **************************
.
.SH SEE ALSO
  orterun(1)
.
